nvme-pci: remap BAR0 to cover admin CQ doorbell for large stride

The existing driver initially maps 8192 bytes of BAR0 which is
intended to cover doorbells of admin SQ and CQ. However, if a
large stride, e.g. 10, is used, the doorbell of admin CQ will
be out of 8192 bytes. Consequently, a page fault will be raised
when the admin CQ doorbell is accessed in nvme_configure_admin_queue().

This patch fixes this issue by remapping BAR0 before accessing
admin CQ doorbell if the initial mapping is not enough.

Signed-off-by: Xu Yu <yu.a.xu@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>